This week I got a support call from one customer: "hey, we have a maintenance scheduled for our Dell servers, is it ok if we shut down the two pre-prod servers?" "what pre-prod servers?" [all the non-production servers were virtualized about 4 years ago] "well, there are two servers here, with pre-prod labels and running Linux; no applications are running on them"

So I guess 4 years ago somebody migrated the servers to a virtual environment and then just forgot about them. The IPs were migrated to the VMs and these servers were left without an IP address on the network connections, so apart from going to the data room and checking every machine (what they did during the maintenance) there was no way to find them. They were still up and running after 4 years.